[caffe2] fix type annotations for workspace.SwitchWorkspace() (pytorch#77464)

Summary: The `create_if_missing` parameter is optional, and defaults to `None`.

Test Plan:
Confirmed that Pyre no longer complains about calling `SwitchWorkspace` with a
single string argument.

Differential Revision: D36366987

Pull Request resolved: pytorch#77464
Approved by: https://github.com/voznesenskym

Author: simpkins

caffe2/python/_import_c_extension.pyi

@@ -179,7 +179,7 @@ def on_module_exit() -> None: ...
 @overload
 def switch_workspace(ws: Workspace): ...
 @overload
-def switch_workspace(name: _PybindStr, create_if_missing: bool): ...
+def switch_workspace(name: _PybindStr, create_if_missing: Optional[bool] = None): ...
 def create_child_workspace(
     parent_ws_name: _PybindStr, child_ws_name: _PybindStr
 ) -> None: ...